47 



" I now come to the consideration of the arguments for the parasi- 

 tism of the Cephalopod of the Argonaut, founded by M. de Blainville 

 on undoubted or admissible facts. The first of these arguments re- 

 poses on the often-repeated statement of the absence of any organ 

 for muscular adhesion in the Cephalopod of the Argonaut. I confess, 

 that when I discovered the Cephalopod of the Nautilus to be fixed to 

 its shell by two strong muscles, and that the corresponding muscles 

 in the Argonaut were very feebly developed, and lost in the mantle, 

 the absence of analogy between the two Cephalopods inclined me, in 

 ■1832, to consider as probable the parasitic theory; subsequently, 

 however, the consideration of the absence of muscular adhesion in 

 the Carinaria, and of any adhesion at all in the Annelides which se- 

 crete shells, deprived this argument of much of its force. 



" Secondly, M. de Blainville observes that 'the muscular integument 

 of the body of the Cephalopod is not thinner than that of the naked 

 species, contrary to that which exists in all conchyliferous MoUusks.' 

 But what Mollusk, we may ask, has its whole body covered with a 

 shell so delicate, so transparent, so flexible and elastic, as is the shell 

 of the living Argonaut* .'' 



" The dorsal border of the mantle is not free," observes M. de 

 Blainville. Granted : and this would be undoubted^ strong proof 

 that the Cephalopod of the Argonaut did not secrete its shell, if it 

 were not provided with other organs for the purpose. In the 

 Pearly Nautilus, on the other hand, which has no veliferous arms, 

 the dorsal border of the mantle is so produced that it can be ex- 

 tended from the involuted spire, which it habitually covers, over 

 the whole exterior of the shell, just as the Argonaut invests its shell 

 with the transparent films of the dorsal pair of arms : the analogy 

 between these two testaceous Cephalopods is perfect as regards their 

 relative position to the shell, but does not extend to their organs of 

 secreting or of adhering to the shell f. 



" The animal does not occupy the posterior part of its shell. This 

 I have ranged in the category of false facts, because the statement is 

 only applicable to the young animal. But granting it were true, as 

 well might we argue the Helix decollata to be a parasite, because it 

 does not, like Magilus, retain and fill with shelly secretion the desert- 

 ed spire of its shells ; or that Magilus was a parasite because it did 



* M. d'Orbigny truly states, " Les coquilles d^ I'Argonaute n'ont pas la 

 contexture vitreuse des Cariiiaires et des Atlantes; elles sont, au contraire, 

 denii-covnees, flexibles; et nous n'en trouvons I'analogue danan^ucun autre 

 des Molhisques." Loo. cit. p. 11. 



t Messrs. de Blainville and Gray conceive me to be in error in the 

 position I have assigned to the Pearly Nautilus in its shell, but their argu- 

 ments on this point are based on the same hasty generalization that has led 

 to the hypothesis of the parasitism of the Argonaut. Judging from the ana- 

 logies which have been cited in support of their views, it would have been 

 equally reasonable to have called in question the accuracy of the relative 

 position which I have assigned to the soft parts of Tere.bratula and Orhi- 

 cula, viz. with the ventral surface applied to one valve, and the dorsal sur- 

 face to the other, because in the Lamellibranchiate bivalves one valve cor- 

 responds to the right, and the other to the left side of the animal. 



